The invention relates generally to lubricating compositions (lubricants) suited to use in marine diesel engines and finds particular application in connection with an additive for boosting the total base number (TBN) of a marine diesel cylinder lubricant and to a lubricating method.
Marine diesel fuels used in low-speed two-stroke engines often contain a large amount of sulfur. The high-sulfur containing diesel fuels produce acidic combustion products, particularly sulfurous and sulfuric acids. These products can be neutralized using marine diesel cylinder lubricants (MDCLs) having a high base content. There are two common measures of basicity that are commonly used in the field of lubricant additives: Total Base Number (TBN), as measured by ASTM D2896, is a titration that measures both strong and weak bases, while ASTM D4739 (BN) is a titration that measures strong bases but does not readily titrate weak bases, such as certain amines. TBN and BN are expressed as an equivalent in milligrams of potash per gram of oil (mg of KOH/g).
Additives that are commonly used to boost the TBN in MDCL engine oils include detergents that are overbased by insoluble metallic salts, such as calcium or magnesium salts. These yield the corresponding metal carbonate as the lubricant is burned in the engine, which is neutralized by the acidic combustion products. However, if a low sulfur fuel is used, some of the metal carbonate is not neutralized and can be deposited on pistons and other engine components, causing wear over time.
One way to address this problem is to use different lubricants depending on the type of fuel being used. The sulfur level allowed in marine fuels in international waters is much higher than that allowed in some coastal areas (referred to as Environmental Control Areas). Therefore, an MDCL with a high TBN level can be used in international waters, while for coastal and inland waters, a low TBN level MDCL is used. However, it can be uneconomic or impractical to carry two MDCLs with different TBN levels.
Another way to address this problem is to use ashless (i.e., low-ash) compounds as TBN-boosting additives. The following relate generally to low-ash additives for lubricants: U.S. Pub. No. 20120040876, published Feb. 16, 2012, entitled ANTHRANILIC ESTERS AS ADDITIVES IN LUBRICANTS, by Preston et al., discloses ester additives. U.S. Pub. No. 20110092403, published Apr. 21, 2011, and U.S. Pub. No. 20140041610, published Feb. 13, 2014, both entitled CYLINDER LUBRICANT FOR A TWO-STROKE MARINE ENGINE, by Lancon et al., disclose oil-soluble fatty amines. WO2014074335, entitled BASIC ASHLESS ADDITIVES, discloses N-hydrocarbyl-substituted γ-aminoesters and aminothioesters.
Since the MDCL is passed through the cylinder only once, it should also have the ability to neutralize the acidic combustion products quickly.
There remains a need for a low-ash additive which boosts TBN of an MDCL while enabling fast neutralization of combustion products.